1. Technological Field
The present invention relates to a semiconductor memory device, and more specifically to a static random access memory (SRAM) including a memory cell which is formed of cross-connected inverters.
2. Description of the Related Art
In recent years, with the higher integration and the lowering of a supply voltage in a semiconductor integrated circuit, an SRAM used in such an integrated circuit has suffered from a problem of an increase of its operational margin defect. In this regard, a disturb margin is an index indicating stability of an SRAM cell at a time when data is read therefrom. The disturb margin shows the capability of a memory cell to hold data without destroying the data when the data is read therefrom. The higher integration causes a variation in threshold voltage among memory cell transistors. Meanwhile, the lowering of a supply voltage causes a variation among the memory cells in timing when a transfer transistor is brought into conduction in response to a rise of the voltage of a word line. As a result, the data held in the memory cell is likely to be inversed and destroyed.
As a method for improving the disturb margin, there is known a method of raising a potential of a word line signal in two steps when reading data (see, for example, “A 1-V TFT-Load SRAM Using a Two-step Word-Voltage Method,” K. Ishibashi, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL 21, NO II, NOVEMBER 1992, hereinafter referred to as non-patent document 1). In addition, another method has been proposed, for example, in which the potential of a word line signal is controlled in accordance with a degree of a variation (global variation) in threshold voltage among memory cells in a memory cell array (see, for example, “A 65 nm SoC Embedded 6T-SRAM Design for Manufacturing with Read and Write Cell Stabilizing Circuits,” S. Ohbayashi, VLSI Circuits Digest of Technical Papers. 2006, hereinafter referred to as non-patent document 2).
The method of non-patent document 1 requires two levels of supply voltages. Accordingly, two types of charge pump circuits have to be prepared. Thus, an increase in a circuit size is a problem.
On the other hand, the method of non-patent document 2 improves a disturb margin by setting the potential of the word line signal to be low when the variation is large. However, a write margin which indicates easiness of writing data is deteriorated. Accordingly, there is a problem that it is difficult to improve both the disturb margin and the write margin at the same time.